LEDs are used in a wide variety of applications from optical communications equipment to digital displays. In communications equipment, LEDs may be used to provide the light source required when propagating optical signal energy from one end of an optical fiber to the other end. In digital displays, for example, LEDs are becoming more pervasive for use in the backlighting that is required for Liquid Crystal Displays (LCD), or similar display units.
LCDs are found in everyday use such as in laptop computers, digital clocks and watches, microwave ovens, CD players, thermostats and many other electronic devices. These devices require displays to communicate pertinent information to the outside world, where LCDs are commonly used because they offer advantages over other display technologies, such as Cathode Ray Tubes (CRTs). Some of the advantages achieved by the LCD over the CRT display are that the LCD offers lighter, thinner design architectures using much less power than the CRT display.
The basic LCD is arranged as layers of polarized glass, electrodes, and liquid crystals, all of which are backlit. As varying voltages are applied to the electrodes of the LCD, the liquid crystals arrange themselves, e.g., “untwist” in the case of twisted nematic (TN) LCDs, in such a way as to allow the backlit light to pass through. Backlighting is required, therefore, in an LCD display to illuminate the design created by the electrically charged liquid crystal molecules.
Various methods are used today to provide the required backlighting for LCDs, including reflective, transmissive, and transflective methodologies. In the transmission and/or transflective categories, a number of different backlighting techniques are used, including incandescent, electroluminescent (EL), fluorescent, LED, and woven fiber optic lighting techniques, to name a few. Incandescent backlights are very bright, but generate a significant amount of undesirable heat. Additionally, the color of the incandescent light is very white, but is highly dependent upon the changing supply voltage.
EL backlighting is based on a solid state phenomenon, which uses colored phosphors to generate light. The main advantages offered by EL backlights include extremely low current requirements, very low heat generation, uniformity, and thinness. One disadvantage to the EL backlighting technique, however, is that an inverter is required, which itself requires up to 50–60 mA of supply current and additional circuit board space.
Fluorescent backlights offer very long lifetimes with low heat generation and low power consumption. Like an EL backlight, fluorescent backlights also require an inverter, but fluorescent backlights are not as sensitive to variations in supply voltage and withstand shock and vibration well.
LED backlighting is a popular choice, especially when relatively smaller LCDs are used. Some of the advantages of LED backlighting include its low cost, long life, and the wide variety of colors that are available. The light provided by the LEDs tends to be rather uneven, however, and a light pipe or light diffuser is often used to create increased uniformity. The forward current supplied to the LED should be regulated, in order to minimize intensity fluctuations due to power supply fluctuations.
As technology progresses, however, the designer is forced to work with increasingly challenging design constraints such as power, weight, and size restrictions. Power supply levels, for example, are particularly challenging with respect to driver circuits for the LED backlights. In particular, many of the electronic systems today are operating with supply voltages in the 3 volt range or less, whereas LEDs used in backlight circuitry, for example, require approximately 3.5–4 volts for proper operation. The designer, therefore, is faced with the arduous task of designing LED driver circuits using power supply voltage levels that offer less than the forward operating voltage required by the LED(s).
One solution to the problem is to provide power supply levels above the operating level of the components used in the particular electronic design. Reduced power supply levels, however, have many advantages for microelectronic design such as reduced quiescent and dynamic power consumption, reduced peak to peak variations in logic levels, and increased speed of operation. The advantages gained by the reduction of the power supply levels often outweigh the advantages gained from using higher power supply levels for driving LEDs, and thus does not provide a practical solution.
Another solution may be to design in other components having reduced power level requirements. The cost of redesign, however, may be prohibitive due to exorbitant component cost or lack of component availability.
Accordingly, there is a need for an apparatus, system and method that allows LED drivers to be used in electronic circuits operating with power supply levels below the specified forward voltage limits of the LEDs. The present invention fulfills these and other needs, and offers other advantages over the prior art.